Method and Device for Altering Repetition Rate in a Mode-Locked Laser

ABSTRACT

A mode locking device is disclosed for altering repetition rate in a mode-locked laser. In an example device, laser light is coupled from a fiber into a cavity through a sliding pigtail collimator with a diameter selected such that it is a close tolerance fit with a female snout on a package. A lens focuses laser light to an appropriate spot size onto a SAM or SESAM, such that back-reflection into the fiber is maximized, A piezoelectric transducer is mounted in cooperation with the SAM or SESAM for cavity tuning.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Provisional PatentApplication No. 62/515,774 filed Jun. 6, 2017 for “Methods for AlteringRepetition Rate of a Pulsed Laser,” of Juan Pino and Eng Hiang Mark Yeo,hereby incorporated by reference in its entirety as though fully setforth herein.

BACKGROUND

Pulsed lasers typically can be categorized as Q-switched, gain-switched,quasi-continuous wave, excimer, or mode-locked lasers. Of these,mode-locked lasers enable extremely short pulses and hence high peakpowers. In addition, each of the longitudinal modes of a mode-lockedlaser provides specific phase relationships with every otherlongitudinal mode, which enables highly coherent systems such as, butnot limited to, frequency combs. Mode-locked lasers typically are saidto include solid state lasers, semiconductor lasers, dye lasers, andfiber lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top-down view of an example mode locking device which may beimplemented for altering repetition rate in a mode-locked laser,

FIG. 2 is a perspective view of the example mode locking device.

FIG. 3 is a cutaway isometric view of the example mode locking deviceshown in FIG. 2 .

FIG. 4 is a diagram of the mode locking device shown in FIGS. 1-3 .

FIG. 5 shows an example of the mode locking device as it may beimplemented in a mode locked fiber laser.

FIG. 6 is another diagram of an example mode locking device.

FIG. 7 is another diagram of an example mode locking device.

FIG. 8 a shows a micro-optic wavelength division multiplexer (WDM)inserted into the free-space section to enable pump light to enter thelaser cavity and gain section.

FIG. 8 b shows the WDM as a dichroic beam splitter that reflects thepump wavelength and transmits the lasing wavelength.

FIG. 9 is another diagram of an example mode locking device.

DETAILED DESCRIPTION

Mode-locked lasers based on doped fibers have a number of advantagesover other laser technologies. Fiber lasers spread the gain out over alarge area, and as a result, tend to not suffer from the sort of thermalbreakdown that solid state lasers are subject to. The fiber itself canbe physically configured in a variety of ways, allowing compact designsthat can be easily incorporated into larger systems. For applicationsrequiring the laser light to eventually enter a fiber, the design isvery practical, with extremely low coupling loss. Polarizationmaintaining (PM) fiber configurations have demonstrated robust modelocking, and are additionally able to withstand mechanical shocks,accelerations, etc.

Mode locking can be achieved actively or passively. In activeconfigurations, an electro-optic effect is periodically modulated at thecharacteristic repetition rate of the laser. For such systems, a“start-up routine” is typically implemented to begin the mode locking,for example, by invoking an algorithm or by a trained user.

Passively mode-locked lasers need a saturable absorbing medium withinthe laser cavity to achieve mode locking. A Semiconductor SaturableAbsorbing Mirror (SESAM) may be provided for turn-key, self-startingmode locked lasers. Prior to introduction of the SESAM, mode lockedlasers had complicated start up routines involving careful tuning ofelectro-optic modulator (EOM) drive frequencies, acousto-optic modulator(AOM) drive frequencies, polarization control, etc.

SESAMs suppress constant wave (CW) lasing by introducingoptical-power-dependent absorption into the laser cavity. As the lasergain is increased, this absorption (as well as other inherent losses)dominates the gain, and the cavity produces amplified spontaneousemission (ASE). As the ASE grows, fluctuations of higher incident poweron the SESAM saturate the absorption of the device, and the overall gainquickly outweighs the loss for shorter and higher intensity pulses, andthe device begins to mode-lock. Now the cavity energy is contained in apulse, allowing the SESAM time to recover. When the pulse travels backto the SESAM the process is repeated.

Self-starting is an important aspect of functional mode locking outsideof the laboratory. Applications outside of the laboratory are enabled bythe ability of the laser to mode lock without complicated algorithms ormanual tuning of alignments or electrical signals.

In a mode-locked laser, the repetition rate (e.g., the temporal rate ofthe laser pulse) is set by the overall cavity length and the groupvelocity of the medium. The group velocity is a difficult parameter totune on the fly, and so the repetition rate is adjusted by changing theoverall cavity length instead. For fiber lasers, cavity lengthadjustment is often accomplished by physically stressing, or pulling, onthe fiber through a piezoelectric transducer (or other mechanicaldevice) glued or otherwise affixed to the fiber itself.

Physically stressing optical fiber suffers from a number of problems.While fiber stretchers are commonplace, the length, type, and componentsin a particular laser oscillator are unique, which limits the ways inwhich most commercial fiber stretchers can be incorporated. In addition,the performance of fiber stretchers is less than optimal in many cases.Large fiber stretchers (e.g., those that affect about 1 cm or more ofthe fiber) inherently have slower response times. This can beproblematic when trying to make a closed-loop system with fast feedback.

Smaller fiber stretchers have much faster response times. However,transferring the full displacement of the piezoelectric transducer tothe laser cavity is imperfect, often losing a factor of about ten in theprocess. Finally, construction of a fast fiber stretcher is difficult,as fiber bends can introduce unwanted resonances, fiber breaks, andadditional cavity loss due to small radius bends in the fibers.

The systems and methods disclosed herein enable a semiconductorsaturable absorber mirror (SESAM) to be mounted onto and displaced by apiezoelectric transducer (“piezo”), thereby changing the repetition rateof a mode-locked pulse laser without the need to stretch the fiber.

Before continuing, it is noted that as used herein, the terms “includes”and “including” mean, but are not limited to, “includes” or “including”and “includes at least” or “including at least.” The term “based on”means “based on” and “based at least in part on.”

FIG. 1 shows an example mode locking device 10 which may be implementedfor altering repetition rate in a mode-locked laser. In an example, themode locking device 10 includes a semiconductor saturable absorbermirror or SESAM 11. The SESAM 11 is mounted on and displaced by apiezoelectric transducer 12.

In an example, the SESAM 11 provides a mechanism for mode-locking alaser. It is an effective mechanism for self-starting a mode-lockedlaser, and simplifies the laser start up routine. In order for the SESAM11 to properly function, the incident fluence (pulse energy/area) of thelaser pulse is well matched to the saturation fluence of the SESAM 11.

For mode locked lasers, an effective way to tune the repetition rate (aswell as the cavity modes) is by physically changing the optical pathlength of the light inside the cavity 18. This can be accomplished byphysically changing the refractive index within the cavity, or byphysically changing the cavity length. Cavity length can be changed bytemperature controlling an element in the cavity to expand or contract,physically stressing an element in the cavity to similarly expand orcontract. Cavity length can also be changed by physical manipulation ofa cavity mirror through piezoelectric transducer 12 or similar device.The device 10 may be implemented for piezo tuning of the cavity lengththrough direct manipulation of the saturable absorbing mirror (SAM).

It is noted that the systems and methods disclosed herein are directedto a fiber laser for purposes of illustration. However, functionally itis a free-space mechanism and therefore is not limited to implementationwith a fiber laser, Other laser technologies may also benefit fromincorporating components and/or operating in conjunction with componentsof the device 10.

An optical fiber 13 provides an optically guided pathway for laser lightto travel into a comb locker package 5. The laser light is coupled intothe cavity 18 through fiber pigtail collimator 14, which collimates thelaser light. The physical diameter of the collimator 14 may be selectedfor a close tolerance fit with the female barrel 15 such that thecollimator 14 may be longitudinally adjusted with high precision withinbarrel 15.

A focusing lens 17 is provided to focus the laser light such that thelight forms an appropriate spot size on the SESAM 11, The focusing lens17 may be selected such that the focused spot of laser light gives afluence appropriate for the SESAM 11. The lens is also manipulated oraligned to maximize back-reflection into the fiber.

The example SESAM 11 is shown in FIG. 1 mounted onto a piezoelectrictransducer 12 in the cavity 18. Should the SESAM 11 itself have need forelectrical connections, the device also enables wire bonding or epoxyconnectors to the package 5 or mounting block 16.

In an example, the maximum displacement of the piezo material of thepiezoelectric transducer 12 may be about 2 microns, with response ratesin the hundreds of kHz. The geometry of the piezo material may beselected such that when the piezoelectric transducer 12 displaces theSESAM 11, the laser light coupling back into the optical fiber 13 isunaffected, and the piezo displacement is within the Rayleigh range ofthe focused spot.

In an example, the retro-reflecting nature of the device 10 may also becompatible with polarization-maintaining geometries. For example, thecomponents in the free-space may be selected to have no birefringence.

In an example, the free-space section in the cavity 18 (e.g., betweenthe collimator 14 and the focusing lens 17) also enables placement ofadditional optical and electro-optic elements, including but not limitedto, optical filters, dichroic mirrors, polarizers, electro-opticmodulators, etc.

The free-space design allows for active, coarse tuning of the cavitylength during construction. In an example, the collimator 14 may be slidin and out of the package 5 while maintaining alignment. The cavityrepetition rate can then be monitored while the laser is mode locked bymonitoring the laser output with a photodiode whose bandwidth is higherthan the laser repetition rate. While actively monitoring the repetitionrate, the length of the oscillator cavity can be controlled by changingthe position of the collimator lens with respect to the SESAM position,and then fixing in place with solder or glue, thus enabling one toprecisely tune the repetition rate to a specific value. In an example,this could be used for constructing a pair of mode-locked lasers withmatched repetition rates thus eliminating the problem of cutting opticalfiber lengths to precise tolerances.

The design of the free-space also allows for active, coarse tuning ofthe cavity length during laser construction. In an example, thecollimator 14 may be slid in and out of the package, while maintainingalignment. The cavity repetition rate can then be monitored as the laseris mode-locked. Furthermore, the cavity repetition rate can be tuned inthis active configuration, enabling laser construction with preciserepetition rates, or constructing a pair of lasers with matchedrepetition rates.

Before continuing, it should be noted that the examples described aboveare provided for purposes of illustration, and are not intended to belimiting. Other devices and/or device configurations may be utilized tocarry out the operations described herein.

FIG. 2 shows an example mode locking device 10. FIG. 3 is a cutawayisometric view of the example mode locking device 10 shown in FIG. 2 .The cutaway provides a better understanding of the spatial relationshipbetween the elements. Particularly visible are the piezoelectrictransducer 12, optical fiber 13, pigtail collimator 14, focusing lens17, conductive traces 19, and dielectric material 20.

The example device 10 shown in FIGS. 2 and 3 , includes a mounting block24 to support the piezoelectric transducer 12 and/or the SAM or SESAM11. It is noted that the mounting block 24 can also support the traces19 and dielectric material 20, or may be made of dielectric material andthus be the same object as the dielectric material 20. Because of itsmass and inertia, the mounting block 24 may also damp vibrations andresonant mechanical modes, thereby increasing the bandwidth of thepiezoelectric transducer 12.

The example device 10 shown in FIGS. 2 and 3 , includes an optical fiber13 in a pigtailed collimator 14. This collimator 14 serves to launch thelight into the mode locking package 5 at an angle substantially normalor perpendicular to the surface of the SESAM 11.

In an example, the launched light is collimated, Collimated lightenables later focusing of the light onto the SESAM 11. Collimated lightalso enables translating the position of the collimator 14 along itslong axis. During the build process, translation of the collimatorenables coarsely tuning the free space optical path length of the modelocking. In an example, the collimator may translate over a distance of3 cm, although other translation ranges may also be provided.

The light is focused onto the SESAM 11 by the lens 17. Thisconfiguration is referred to as a cat's eye retroreflector. Being lessprone to alignment errors, this configuration enables the mode lockingdevice 10 to perform its functions compactly, efficiently, and reliably.

In an example, the mode locking device 10 provides for a symmetricdesign. That is, the incident and reflected light both travel along thesame path. This configuration also enables efficient retroreflection andback coupling of the light into the fiber 13. Coupling efficiency isimportant, as increased loss adversely affects laser performance.

Another benefit of the cat's eye configuration is the ability totranslate the position of the SESAM 11 without affecting the couplingefficiency. When the focusing lens 17 is selected such that the SESAMtranslation is smaller than the Rayleigh range of the focused beam, theretro-reflected beam is mostly, if not completely, insensitive tochanges in the position of the SESAM 11.

It is noted that in another example, the SESAM 11 may be replaced with asaturable absorber mirror (SAM) without fundamentally altering theoperation of the mode locking device 10.

In an example, piezoelectric transducer 12 is controlled by a voltageapplied between conductive traces 19, which may be patterned onto adielectric material 20. The traces 19 may be gold, silver, aluminum, orany other conductive material suitable for applying the voltage,Dimensions of the traces 19 are important only insofar as the traces 19do not overheat or interfere with one another. The dielectric material20 may be polymer, ceramic, or any other patternable, electricallyinsulating material.

FIG. 4 is a diagram of the mode locking device 10 shown in FIGS. 1-3 .During operation, the mode locking device 10 may include translating theposition of the SAM or SESAM 11 back and forth (e.g., as illustrated byarrows 21) along the mirror travel range. This adjusts the free spacedistance 22 to tune the optical path length of the mode locking device10. Similarly, the collimator 14 can translate back and forth (e.g. asillustrated by arrows 23) along the collimator travel range to tune theoptical path length of the comb locker device 10.

The design of the mode locking device described herein is fullycompatible with a polarization-maintaining fiber system. That is, when apolarization-maintaining fiber system is provided, the retroreflectedbeam has substantially the same polarization as the incident beam,allowing for a simple design and geometry because the polarization axisof the incoming light does not need to be aligned to any part of themode locking.

In this design, the cat's eye retroreflector is formed by thecombination of the focusing lens 17 and the SAM or SESAM 11. The lens 17bends the collimated light 25 from the collimator 14 into focused light26 that impinges onto the SESAM 11. If the focus of the focused light 26occurs at the surface of the SESAM 11, the reflected light from theSESAM 11 can be aligned to counter-propagate with nearly the samespatial mode as the input light and thus achieve good couplingefficiency back into the optical fiber. In this manner, the opticalpower losses in the mode locking device 10 are minimized, which helps inachieving good lasing performance.

The diameter of the beam of collimated light 25 (e.g., as determined bythe collimator), and the focal length of the focusing lens 17, determinethe spot size of the light focused light 26 on the SESAM 11. Twocriteria for the spot size should be simultaneously met. One criterionincludes minimizing coupling losses. To minimize coupling losses, thedesired mirror travel range 21 must be less than the Rayleigh range ofthe focused light 26 (which is proportional to the square of the spotdiameter of the focused light 26), Another criterion includes the spotdiameter of the focused light 26 on the SESAM 11. The spot size shouldbe selected so that stable mode locking is achieved. The exact spot sizedepends on several factors, among these being the saturation fluence(pulse energy per area) of the SESAM 11, the gain and loss in the fiberlaser cavity 18, and the desired output pulse shape from the laser.

The spot size influences whether stable mode locking occurs at all. Thespot size also influences how short the pulse is (and therefore how widethe bandwidth of the output light is).

The travel range of the mirror comes from two aspects. In order tominimize optical losses, the retro-reflected light off the SESAM 11should have the same spatial mode profile as the light first exiting thecollimator 14. This maximizes the amount of light recoupled into thecollimator 14, thus reducing overall loss, and improves the lasingefficiency and minimizes phase noise in the laser modes. In normaloperation, the focus is near the face of the SESAM. A length scale tocompare the distance between the focal plane and the SESAM face is theRaleigh range where distances below the Raleigh range leads to efficientcoupling and distances comparable to and larger than the Raleigh rangewill lead to poor coupling.

In addition, the intensity dependent reflection coefficient of the SESAM11, depends on the fluence of the circulating pulse. As the fluencescales inversely with spot size on the SESAM face, the ideal conditionwould occur again when the focus of the beam coincides with the SESAMface. When the distance of the focal plane to the SESAM face is smallcompared to the Raleigh range, the spot size is very close to idealwhereas if the distance of the focal plane to the SESAM face iscomparable or larger to the Raleigh range, then the spot size will belarger. Therefore, much higher pump powers may be required to attainmode-lock.

FIG. 5 shows an example comb locker device 10 as it may be implementedin a mode-locked fiber laser. In this example, the overall repetitionrate of the mode-locked laser is set by the overall length of the lasercavity, which is the sum of the free space distance 22 and the fiberlength 27, divided by the average round trip group velocity. For alinear cavity, the repetition rate is given by the following formula:

$\begin{matrix}{F_{rep} = \frac{c}{2\left( {{n_{g.{air}}L_{air}} + {n_{g.{fiber}}L_{fiber}}} \right)}} & {{EQN}1}\end{matrix}$

Where n_(g.air) and n_(g.fiber) are the group indices of air and fiberrespectively. L_(air) and L_(fiber) are the physical lengths of the freespace distance 22 and the fiber length 27. The group velocities in airand fiber are given by v_(g.air)=c/n_(g.air) and v_(g.fiber)=c/n_(g.air)so that the repetition rate can be rewritten as:

$\begin{matrix}{F_{rep} = \frac{1}{2\left( {\frac{L_{air}}{v_{g.{air}}} + \frac{L_{fiber}}{v_{g.{fiber}}}} \right)}} & {{EQN}2}\end{matrix}$

To tune the overall repetition rate, the user must control one of thesetwo parameters (i.e., overall length, or average round trip groupvelocity). Controlling the group velocity can be accomplished bytemperature tuning the laser fiber. But this is a slow effect and alsoaffects the offset frequency of a frequency comb built using thissystem. The user can more quickly tune the fiber length with a fiberstretcher as discussed, but this also has drawbacks.

Instead, the systems and method disclosed herein control the free spacedistance 22. During laser builds, the mode locking design can alsocoarsely tune the length of the distance 22 and free space bytranslating the position of the collimator without affecting the fibercoupling, or mode locking characteristics of the laser. This isespecially important for systems where the repetition rate must be aprecise value. The method includes coarse tuning of the repetition ratewhile actively monitoring the repetition rate of the laser whileenergized.

Before continuing, it is noted that the operations shown and describedherein are provided to illustrate example implementations. It is notedthat the operations are not limited to the ordering shown. Still otheroperations may also be implemented.

By way of illustration, a further example may include a robustfree-space section within a fiber laser cavity, made possible by thecat's eye retro-reflector topology. As will be readily appreciated bythose having ordinary skill in the art after becoming familiar with theteachings herein, this implementation enables many advantageousvariations.

Example implementations of the mode locking device 10 are describedbelow.

Temperature tuning with thermo-electric cooler (TEC). In an examplebased on design considerations, the mode locking device 10 enablestemperature control of the SESAM independent of cavity length. A heatingelement, or thermoelectric element, can replace, or be positionedalongside the mounting block, or be positioned directly on the end ofthe piezoelectric material. This enables tuning the Bragg grating ofSESAM 11 by altering the index and spacing of the thin-film Bragg stacksof the SESAM. In this manner, temperature tuning of the SESAM canprovide independent, orthogonal control over the carrier-envelope phaseof the pulsed output of the mode-locked laser, which is important foroptical frequency comb applications. Similar behavior has been observedby using fiber Bragg gratings. Independent temperature control of themode locker cavity 18 can be achieved by thermally contacting a TEC 30to an outside surface of butterfly case of the mode locker 10.

Free space filters (polarization, spectral). In an example based ondesign considerations, the the mode locking device 10 also enables acollimated free space section within the laser cavity. FIG. 6 is anotherdiagram of an example mode locking device illustrating an example ofcollimated free space 32. Optical elements may be provided to improve oralter laser performance. These elements may include, but are not limitedto, optical filters to attenuate optical wavelengths that are unwantedwithin the laser cavity, such as high-pass, low-pass, bandpass, andnotch filters. These elements may also include, but are not limited to,polarizers to attenuate unwanted polarizations within the cavity.

For example, high-bandwidth, electro-optic intensity modulators such asgraphene modulators or polarization altering devices in conjunction withintracavity polarization loss or polarizers such as Pill PMN-PT orPZN-PT crystals, in a general class of materials known as“OptoCeramics,” can be used in the free-space section to rotate thepolarization into the non-lasing polarization state creatingelectronically controlled loss with very high bandwidth over a thinoptical element. These are useful for modulation of the carrier envelopeoffset frequency or phase.

Electro-optics. In an example based on design considerations, the freespace section within the mode locking device 10 may also incorporateelectro-optic elements within the cavity 18. An example may include bulkmedia, and/or waveguide devices, Example electro-optics may includephase modulators (e.g., electro-optic modulators, liquid crystals),frequency modulators (e.g., acousto-optic modulators), polarizationrotation (e.g., liquid crystals, electronically controlled Faradaymaterials), and intensity modulators (e.g., crossed polarizers, beamdeflectors), to name only a few examples.

Pump and second fiber. FIG. 7 is another diagram of an example modelocking device with dichroic mirror 28 and pump light collimator 29 forproviding collimated pump light. FIG. 8 a illustrates gain medium 1,passive fiber 2, 3, output coupler 4, pump input 5, comb locker with WDM6, and fusion splices 7. FIG. 8 b illustrates gain medium 8, passivefiber 9, 10, 11, output coupler 12, pump input 13, comb locker 14,fiberized WDM 15, and fusion splices 16.

FIG. 8 a shows a micro-optic wavelength division multiplexer (WDM)inserted into the free-space section 18 to enable pump light to enterthe laser cavity and gain section. In an example configuration, the WDMis a dichroic beam splitter 28 that reflects the pump wavelength andtransmits the lasing wavelength. This configuration eliminates the needfor fiberized WDM (FIG. 8 b ) inside the laser cavity and has severaladvantages. Examples include, but are not limited to, the laser cavitycan be made smaller allowing for higher pulse repetition rates. Inaddition, the design is more flexible and can allow a wider variety ofpump laser wavelengths. Pump light can be prevented from illuminatingthe SAM or SESAM, which can be important for high-energy pumpwavelengths. The losses inherent with a fiberized WDM are reducedthereby reducing phase noise in the laser.

Multiple PZT transducers. In an example based on design considerations,multiple piezoelectric transducers may be provided to achieve both largedynamic range and high bandwidth response. In an example, multiplepiezoelectric transducers could also be stacked on top of each other.FIG. 9 is another diagram of an example mode locking device showing alarger, slower tuning piezoelectric transducer or section 12 a providinga longer stroke, and a smaller, faster tuning piezoelectric transduceror section 12 b for tighter control over the repetition rate.

Direct focusing to eliminate the free-space section of collimated light,An example simpler optical layout for the mode locking device mayinclude replacing the focusing lens and collimator with a singlefocusing lens located near or attached directly to the end of the inputoptical fiber. For example, a ball or GRIN lens can be glued orotherwise attached directly to the fiber to achieve properly focusedlight without need for collimation.

These and other examples are also contemplated. It is noted that theexamples shown and described are provided for purposes of illustrationand are not intended to be limiting.

1. A method of altering repetition rate in a mode-locked laser with amode locking device, comprising: coupling laser light from a fiberthrough a pigtail collimator into a cavity; focusing the laser light toan appropriate spot size onto a SAM or SESAM, wherein fluence at afocused spot closely matches saturation fluence of the SAM or SE SAM;aligning the lens to maximize back reflection into the fiber; andchanging a position of the SAM or SESAM so as to change an optical pathlength of a laser cavity.
 2. The method of claim 1, further comprisingfree space aligning of the focused laser light onto the SAM or SESAM. 3.The method of claim 1, further comprising mounting the SAM or SESAM ontoa piezoelectric transducer to tune the cavity.
 4. The method of claim 3,further comprising providing a large stroke measured in hundreds ofkilohertz and fast response while producing a useful stroke in a rangeof 1-2 um for long-term locking.
 5. The method of claim 1, furthercomprising a cat's-eye retro-reflector configuration aligning the laserlight onto the SAM or SESAM to achieve robust fiber coupling.
 6. Themethod of claim 1, further comprising providing electrical connectionsto the SAM or SESAM while mounted on the piezoelectric transducer. 7.The method of claim 1, further comprising providing one or more opticalelements in a light path of the mode locking device, the opticalelements selected from high-pass filters, low-pass filters, bandpassfilters, notch filters, attenuators, electro-optic modulators,polarizers, bulk media, and waveguides.
 8. The method of claim 1,further comprising a dichoic mirror in a freespace section of the lasercavity to effectuate wavelength division multiplexing (pump lightcoupling).
 9. The method of claim 1, further comprising tuning of anoptical pathlength of the cavity by allowing the pigtail collimator totranslate over a given range within the cavity.
 10. The method of claim1, further comprising providing a polarization-maintaining fiberconfiguration for the laser light.
 11. The method of claim 1, furthercomprising providing independent control of the temperature of the SAMor SESAM.
 12. A mode locking device for altering repetition rate of amode-locked laser, comprising: a piezoelectric transducer controlled byan applied voltage; a SAM or SESAM in association with the piezoelectrictransducer, wherein translating a position of the SAM or SESAM back andforth along a mirror travel range adjusts the free space distance totune the mode locking device; an optical fiber in a collimator, thecollimator configured to launch light at an angle substantially normalor perpendicular to a surface of the SESAM, wherein translation of thecollimator enables coarsely tuning a free space optical path length ofthe mode locking device; and a focusing lens to focus the light onto theSAM or SESAM.
 13. The mode locking device of claim 12, wherein thefocusing lens is selected such that translation of the SAM or SESAM issmaller than the Rayleigh range of the focused light so that theretro-reflected beam is mostly, if not completely, insensitive tochanges in the position of the SAM or SESAM.
 14. The mode locking deviceof claim 12, further comprising at least one of a high-pass filter,low-pass filter, notch filter, bandblock filter, attenuator,electro-optic modulator, polarizer, bulk media, and waveguide.
 15. Themode locking device of claim 12, further comprising a dichroic mirror inthe free space section of the cavity to effectuate wavelength divisionmultiplexing.
 16. The mode locking device of claim 12, furthercomprising a polarization-maintaining fiber configuration for the laserlight.
 17. The mode locking device of claim 12, further comprisingeither or both of a thermoelectric cooler and a heating element tocontrol a temperature of the SAM or SESAM.
 18. A mode locking device foraltering repetition rate of a mode-locked laser, comprising: apiezoelectric material controlled by an applied voltage; a SAM or SESAMin association with the piezoelectric material, wherein translating aposition of the SAM or SESAM back and forth along a mirror travel rangeadjusts the free space distance to tune the mode locking device; anoptical fiber in a collimator, the collimator configured to launch lightat an angle substantially normal or perpendicular to a surface of theSESAM, wherein translation of the collimator enables coarsely tuning afree space optical path length of the mode locking device; a focusinglens to focus the light onto the SAM or SESAM; and a mounting block tosupport the piezoelectric material and/or the SAM or SESAM, the mountingblock dampening vibrations and resonant mechanical modes, therebyincreasing the bandwidth of the piezoelectric transducer; wherein acat's eye retroreflector configuration translates a position of the SAMor SESAM without affecting the coupling efficiency.
 19. The mode lockingdevice of claim 18, wherein the piezoelectric material provides a largestroke measured in hundreds of kilohertz and fast response whileproducing a useful stroke in a range of 1-turn for long-term locking.20. The mode locking device of claim 18, further comprising: at leastone of a polarization-maintaining fiber configuration, a heating elementon the SAM or SESAM, a thermoelectric cooler on the SAM or SESAM; andone or more optical elements selected from high-pass filters, low-passfilters, bandpass filters, notch filters, attenuators, dichroic mirrors,electro-optic modulators, polarizers, bulk media, and waveguides.